The trial is based on preliminary data showing that upright tilt table testing can provoke a drop in blood pressure consistent with neurally mediated hypotension (NMH) in a high proportion of those with chronic fatigue syndrome (CFS), and that unblinded treatment of the NMH leads to an improvement in CFS symptoms in 40-70% of CFS patients. The specific aim of this study is to determine whether patients aged 18 to 50 years with CFS and NMH will have a greater improvement in (1) self-reported general sense of well being and (2) objective orthostatic tolerance when treated with fludrocortisone than when treated with placebo. Eligible subjects are randomized to receive either fludrocortisone (escalating to 0.1 mg/day) or placebo for nine weeks. In week 8-9 of treatment, subjects undergo repeat tilt testing. The primary outcome measure is the proportion with a 15 point improvement in wellness on a 100 point wellness score, and a secondary outcome is the proportion with improvement in the number of minutes before the development of hypotension during upright tilt.